Non-specific mechanisms of potentiating host cell mediated immunity (CI) is used for immunotherapy of a number of tumors. Currently three agents, the Ribi vaccine, a methanol extract residue (MER) of BCG and Listeria monocytogenes (LM) are being used for this purpose. The latter organism is mainly used in vivo and in vitro to probe for mechanism of non-specific killing of tumor cells. By selective depletion and purification methods, attempts are being made in identifying the effector cells involved and their mode of action. For example do cells interact or synergise during non-specific tumor killing? Are there mediators involved? Do such mediators recruit other cell components or act independently? What is their physiological properties and chemical nature? These and other questions are being asked for better understanding and safer approaches to non-specific immunotherapy of malignant tissue.